340 in 63 dart

With a little creativity, Magnum manifolds can be made to fit, but with manual steering only. The right side is pretty much tight against the inner fender. A couple strategically placed dents and motor mount shims should help, as would having 1/8" - 1/4" milled from the manifold port face. The left side requires shortening the outer steering column tube and replacing the pot coupling with a u-joint. Even then a bit of grinder work on the manifold is required. Like the right side, having the port face milled will help. The early large outlet Magnum manifolds flow as good as any cast iron, and can be had fairly cheaply.

There's just no cheap + easy + good flowing solution for early A bodies. You get 2 out of 3.
Cheap + easy (OEM 64-66 manifolds) flow poorly.
Cheap + good flow (Magnum manifolds or fender well headers) some hassle to fit and hook to exhaust system.
Easy + good flow (TTI, Dougs or early 340 manifolds) not exactly cheap!

What power level are you expecting from your 340?